(1) Field of the Invention
The present invention relates to an erasable storage apparatus, such as an optical magnetic disk unit in which a bias magnetic field is switched between an erase state and a record state, and a bias control method therefor. Further, the present invention can be applied to apparatuses which use recording media in the form disks, cards and tapes. Storage units according to the present invention are connected to host computers such as personal computers and so forth.
For example, there are three types of optical disks. The first type is a read-only type in which data is previously recorded during production and no data can be recorded thereafter. The second type is a one-time recording type in which data cannot be erased from disk. The third type is a rewritable type in which data can be erased and new data can be recorded in the same area. The present invention particularly relates to an optical magnetic disk apparatus which uses an optical magnetic disk of the third type. In the specification, a data reproduction operation may be called a data read operation, and a data record operation may be called a data write operation.
(2) Description of the Related Art
In a rewritable optical magnetic disk apparatus, when data is written or erased, a bias magnetic field is applied and laser light is projected onto the optical magnetic disk at the same time. When data is read, only laser light projection is performed. A bias magnetic field perpendicular to the surface of the medium is applied when data is written, and a bias magnetic field perpendicular to the surface of the medium but with a reverse polarity to that of the field used when data is written, is applied when data is erased.
When a data rewrite operation is performed, a data erase operation is performed on areas of the medium on which new data is to be written before the data is written, then, data is written in the areas. Further, it is verified that data is correctly written. Namely, in the data rewrite operation, it is necessary to sequentially perform an erase operation, a write operation and a verify operation. Therefore, the above-mentioned switching of the bias magnetic field is always performed when data is rewritten.
However, this switching of the bias magnetic field requires a relatively long time. The time necessary to establish a new bias magnetic field may be called a bias-set-up time.
In the optical magnetic disk apparatus, rewrite operations are carried out in units of sectors or of blocks, therefore, the polarity of the bias magnet is frequently changed during a rewrite operation. Therefore, there occur problems due to the bias set up time.
For example, when the optical magnetic disk apparatus receives write data from a host computer, the time necessary to transfer the write data to a buffer memory of the apparatus is very short relative to a total time for the write operation. Further, when data is written into continuous areas, a seek time to place the head to a target position on the disk is also short because the distance the head is moved is short. In this way, the bias-set-up operation is the longest operation in the data rewrite operation. Further, the bias-set-up time is constant regardless of the position of the head.
When data is rewritten, the head seek operation head and the switching of the bias magnet to the erase state start simultaneously. After the head moves to a target position, the erase operation, the write operation and the verify operation are sequentially carried out. However, since the bias-set-up time is long, a problem occurs that although the head is already at the target position, the erase operation cannot start because the bias magnetic field is not yet set up in the erase state. If the bias-set-up operation is finished directly after the target position passes under the head, the erase operation must wait until the target position passes under the head again.
Namely, in the conventional optical magnetic apparatus, the bias-set-up time determines the access time for the rewrite operation, and the time for the rewrite operation is increased due to the bias-set-up time.
In recent years, the speed of the spindle motor which rotates the optical magnetic disk has increased in order to improve the access time and the slow access time due to the bias-set-up time has become a serious problem.